1. Field of the Invention
The present invention relates to a substrate for an information recording medium, an information recording medium and a process for producing an information recording medium. More specifically, the present invention relates to a substrate for an information recording medium which substrate has high infrared irradiation-heating efficiency, an information recording medium comprising a multi-layered film having an information recording layer formed on the substrate, and a process for producing an excellent information recording medium which comprises keeping the substrate in a temperature state suitable for forming the multi-layered film.
2. Related Art Statement
As a substrate for an information recording medium such as a magnetic disk, conventionally, there is used a substrate made of a lithium-containing aluminosilicate glass having a high Young's modulus (e.g., JP-A-2001-180969) or a substrate made of a crystallized glass having a crystal layer precipitated by heat treatment of a glass having a specific composition (e.g., JP-A-2000-119042).
The information recording medium is produced by forming a multi-layered film including an information recording layer on the above substrate. When the multi-layered film is formed on the above substrate, for example, the substrate is first introduced into a substrate-heating zone of a film-forming apparatus and heated up to a temperature at which a film is formable by sputtering. After the temperature of the substrate is sufficiently increased, the substrate is transferred to a first film-forming zone, and a layer corresponding to the lowermost layer of a multi-layered film is formed on the substrate. Then, the substrate is transferred to a second film-forming zone, and a layer is formed on the lowermost layer. The substrate is transferred along film-forming zones toward a backward end zone to form layers as described above, whereby the multi-layered film including an information recording layer is formed on the substrate. Since the above heating and film formation are carried out under a low pressure attained by gas exhaustion with a vacuum pump, the above heating is inevitably performed by a non-contact method. Heating by radiation is therefore suitable for heating the substrate.
It is required to perform the above film formation before the temperature of the substrate comes to be lower than the temperature suitable for the film formation. When the layer formation takes too long, the temperature of the substrate decreases, and there is caused a problem that it is difficult to attain a substrate temperature sufficient for forming a layer in a film-forming zone positioned at a later stage. For keeping the substrate at a film-formable temperature for a long period time, it may be thinkable to heat the substrate to a higher temperature. When the substrate heating rate is small, it is required to increase the heating time period, and it is also required to increase the residence time period of the substrate in the heating zone. The residence time period of the substrate in each film-forming zone comes to be longer, and it is no longer possible to maintain the sufficient substrate temperature in the film-forming zone at a later stage. Further, there is no way to improve a throughput, either.
For increasing the heating rate, there may be possibly employed a means of irradiating the substrate with light having higher power. However, the problem is that a substrate made of a glass or a substrate made of a crystallized glass has low light absorption, so that it is difficult to attain sufficient heating efficiency.